Enhanced Strength and Ductility at a Cryogenic Temperature in a Nanostructured Ni-Fe Alloy

Abstract:

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The compressive behavior was investigated on an electrodeposited nanocrystalline
Ni-20%Fe alloy with a grain size of about 22 nm at room temperature (RT), 298 K, and the
liquid nitrogen temperature (LN2T), 77 K. The sensitivity of the yield strength and plastic strain
to the test temperature at different grain sizes was discussed. Moreover, through the
Transmission Electron Microscopy (TEM) examination and microhardness measurement, the
microstructures before and after the compression test at RT and LN2T were studied.

Abstract: Indentation methods have been used for the study of the hardness and deformation
characteristics MoSi2. Micro-nanoindentation tests at loads from 10 mN to 2000 mN were carried out using the depth-sensing method. Measurements of the microhardness using conventional Vickers method was carried out at loads of 500 mN, 1000 mN and 2000 mN. The Universal (Martens), Plastic and conventional Vickers hardness values were calculated at different indentation loads. Evident indentation load - size effect was found in both materials. According to the results,
the pre-strain reduces the micro-nano hardness values, probably due to the activation of slip systems during the high-temperature deformation.

Abstract: In the present work high pressure torsion (HPT) was imposed on commercial purity (CP)
tungsten at different temperatures of 450 °С and 490 °С to achieve different microstructures and grain
boundary misorientation spectra (GBMS). After HPT at 450°С, ultrafine grained microstructure with
an average grain size of ~150 nm was developed in the metal. HPT at 490 °С results in an elongated
structures with average width of ~500nm. EBSD investigations showed that over 92% are HAGB
type in microstructure HPT-processed at 450°С, and in contrast, over 50% of grain boundaries are
LAGB in sample processed at 490°С. Annealing at 900°С for 1h, of the sample with homogeneous
UFG structure resulted from HPT at 450°C, leads to only limited decrease (~20%) in microhardness.

Abstract: Evolution of structure of high-purity and commercially pure copper at severe plastic deformation (SPD) by high pressure torsion (HPT) at room temperature and in liquid nitrogen has been studied by transmission electron microscopy (TEM) and measurements of microhardness. Thermal stability of structure obtained by HPT has been investigated. Factors preventing from obtaining nanocrystalline structure in Cu are analyzed and possible ways of their overcoming are discussed.

Abstract: Disks of as-extruded Mg-4Nd alloy were processed by high-pressure torsion (HPT) through ¼ to 5 turns at room temperature. The first 1/4 turn of HPT induces large numbers of twins and some dislocation tangles in the center region of the disk. With increase of torsional straining, the twinning is inhibited gradually and the dislocation density increases relating to the formation of dislocation substructures and ultimately transforming to high fractions of equiaxed gains which have an average grain size of ~200 nm and high-angle boundaries. HPT significantly improves the values of microhardness of this alloy. The hardness values in both the central and edge regions show a sharp rise after HPT for 1/4 turn and exhibit nearly saturation after 1/2 turn although there is a trend of a slight increase with increasing numbers of turns. The experimental results suggest more homogeneous microstructures may be produced by larger numbers of turns in the HPT process.

Abstract: Microstructure evolution during annealing of a Ni-20%Cr alloy subjected to high-pressure torsion (HPT) at ambient temperature was examined. It was shown that discontinuous static recrystallization (DSRX) occurs in non-uniform manner under subsequent annealing in the alloy strained to ε<4. The material strained to ε6 or higher exhibits continuous grain growth (GG) under subsequent annealing. It is attributed to the fact that HPT led to the formation of nanoscale grains with an average size of 50 nm. Increasing fraction of these grains with strain leads to transition from DSRX to continuous GG under subsequent annealing. It was found that the main feature of recrystallization behavior of the Ni-20%Cr alloy subjected to HPT is simultaneous occurrence of short-range ordering and recrystallization processes under annealing conditions. As a result, despite the formation of recrystallized structure with an average grain size of 340 nm after annealing at 600°C (0.52 Tm), the material exhibits very high microhardness of 3.6 GPa.